Devices such as the home telephone derive their electrical current needs from the telephone line. As telephone devices become more complicated, their electrical current needs increase until they can no longer be served by the limited current available from a telephone line. Also, FCC regulations have established stringent criteria for connection of devices to the telephone network. These criteria require, among other things, that the device must operate with as little as 20 milliamperes based on the range of voltage supplied by the telephone company, the range of telephone line impedance and the required off-hook device impedance. Thus, auxiliary power derived from available power lines, usually 110-volts a.c. in the United States, is often utilized. Examples of such devices requiring auxiliary power are key telephones for businesses or pay telephones owned by telephone companies, vending companies, or private individuals and businesses. In addition, several sophisticated home telephone products have recently been introduced containing microprocessor circuitry providing varied features.
The use of auxillary power has many disadvantages. It encourages inefficient design of devices that operate over telephone lines resulting in increased operating costs for the a.c. power consumed. Installation costs are also increased due to the need for costly skilled labor to install the device and because the device may be located far from a power line or otherwise inconveniently located for such connection. Also, hazardous conditions may occur due to the potential for electrical shock and tampering by unauthorized persons. Indeed, the above-mentioned FCC regulations also expressly prohibit placement of hazardous voltages or currents on the telephone line.
In addition, some devices, such as pay telephones, may depend so completely on the availability of auxiliary power that they become inoperable in the event of a power outage, loss, or failure. Thus, in an emergency situation where power is out, a telephone so affected would be unavailable for reporting the emergency. Also, memory devices within the telephone device which are dependent on the auxiliary power would lose their contents and, as a result, would require additional servicing when power is resumed. This will result in higher maintenance costs and loss of use of the device until it can be serviced.
Previous failed attempts to power devices directly from the telephone line without auxiliary power have used current limiting devices connected to the telephone line in combination with common switching regulators for each output voltage required. This approach has not proved successful due to the limited efficiency of switching regulators (on the order of 60-75%), and their inability to operate over the entire range of telephone company supply voltages, currents and line impedances. In addition, by using this approach, no electrical isolation is provided between the telephone line and the telephone circuitry.